1. Field of the Invention
The present invention relates to voltage generators and more particularly to voltage generators suitable for generating accurate voltages for use in the tuning control circuitry of radio or television apparatus.
At present most T.V. tuners use digital frequency synthesis to generate the local oscillator varactor voltage for obtaining the correct LO frequency. This synthesiser is frequently required to function up to 1.3 GHz., and a known design uses a single chip synthesiser with external high voltage components.
This synthesised voltage as well as driving the LO varactor is also required to drive the varactors in the image rejection filters prior to the down-conversion stage. Since these filters must accurately track with the LO then the varactors must be well matched to each other which is a costly and time consuming task.
2. Description of Related Art
Present proposed systems for overcoming these problems utilize a procedure called auto-alignment. In this system the varactors, which undergo only a very coarse selection procedure, are each driven by their own high voltage line therefore an error voltage can be introduced so aligning each varactor accurately. The required error voltage can be easily monitored and recorded during alignment procedure and then stored and used as required by the microprocessor which controls the digital synthesis loop, a procedure known as static auto-alignment; or the required error voltage can be set up on each reprogram of the tuner by a series of dynamic algorithms, controlled by the microprocessor, the so called dynamic auto-alignment procedure.
A typical system would use the synthesis loop as at present to generate the LO varactor voltage. The prefilter varactor voltages are then obtained by adding an error voltage +dV to the synthesised voltage V, which is normally in the range 0 to 30V.
For optimum integration of the synthesiser system these high voltage outputs and their associated voltage differencing circuitry need to be integrated with the high frequency processing circuitry.
This is disadvantageous since the high frequency process required to produce the 1.3 GHz., synthesiser will inherently have a low breakdown voltage e.g. for a typical process BV is approx. 10V, which is much lower than that required by the high voltage sections.